1. Field of the Invention
The present invention relates to an input apparatus having an electrostatic sensor. More specifically, it relates to an input apparatus having an electrostatic sensor that detects the capacitance generated by a human appendage, such as a hand or finger, approaching or contacting the sensor and that outputs an electrical signal.
2. Description of the Related Art
FIGS. 6A and 6B show part of a typical electrostatic sensor. FIG. 6A is a plan view of an electrostatic sensor. FIG. 6B shows output signals of the electrostatic sensor.
As shown in FIG. 6A, the electrostatic sensor 50 has two electrodes 51 and 52 for detecting capacitance. The surface of the electrodes 51 and 52 is covered with an insulating sheet. In FIG. 6A, however, the insulating sheet is not shown.
A predetermined voltage is applied to the electrodes 51 and 52. When a moveable body H, such as the user's finger or an input pen, contacts the surface of the insulating sheet covering the electrodes 51 and 52, a capacitance C is generated between the electrodes 51 and 52 that oppose the moveable body H. When the area where the moveable body H and the electrode 51 face each other is S1, the area where the moveable body H and the electrode 52 face each other is S2, the distance between the moveable body and each of the electrodes is d, and the permittivity is ε, the capacitance C1 generated between the moveable body H and the electrode 51 is C1=ε·S1/d, and the capacitance C2 generated between the moveable body H and the electrode 52 is C2=ε·S2/d. Moving the moveable body H in the X1 or X2 direction, shown in the drawing, changes the areas S1 and S2. For this reason, the capacitances C1 and C2 can be changed.
The electrostatic sensor 50 has a C/V converting means (not shown in the drawing) for converting the capacitance C into voltage. The C/V converting means enables the electrostatic sensor 50 to detect the capacitances C1 and C2, which change with time due to the movement of the moveable body H.
When the moveable body H, such as a finger, moves in the X2 direction, shown in the drawing, signals V1 and V2 are as shown in FIG. 6B. Here, V1 is the voltage signal converted from the capacitance C1 by the C/V converting means and V2 is the voltage signal converted from the capacitance C2 by the C/V converting means.
When a controller, not shown in the drawing, first detects the signal V1 and, then, the signal V2, the moveable body H is detected to be moving in the direction from X1 to X2, as shown in the drawing. On the contrary, when the signal V2 is detected first and the signal V1 is detected next, the moveable body H is detected to be moving in the direction from X2 to X1, as shown in the drawing.
The controller, for example, controls the movement of a cursor on a display, based on the detected information.
If the electrodes 51 and 52 are shaped so that they, for example, gradually increase (or gradually decrease) in size in the moving direction of the moveable body, only one electrode may be required for the sensor.
For such known electrostatic sensors, the size of the electrodes is large. Therefore, the change in capacitance of the electrostatic sensor generated when the moveable body, such as a finger, contacts the sensor is large, and, thus, the sensor is highly sensitive.
However, recent demands for reducing the size of the electrodes of electrostatic sensors and increasing the thickness of the case have arose from the demands for reducing the size and increasing the functionality of input apparatuses. For reduced-size electrostatic sensors, it is difficult to detect an input by contact of the moveable body since the capacitance of the sensor is small.
A typical portable input apparatus intermittently drives input means having an electrostatic sensor in order to reduce the electrical power consumption. Specifically, when the input apparatus is not used and is in a standby mode, a driving pulse (clock signal) for driving the electrostatic sensor is not generated, but, when the input apparatus is put to use, the driving pulse is generated.
When the input apparatus having an electrostatic sensor in a standby mode is started up by a moveable body, such as a finger, contacting the sensor, the time required for start-up is about 300 ms. Thus, the responsiveness for the initial operation of the sensor is not good, making the sensor unsuitable for intermittent driving.